Analog signal processing is used in many electronic systems, and typically the analog signal processing uses circuits for amplifying analog signals. An example of such electronic systems is a direct conversion receiver for radio frequency signals, which is the most popular receiver type in mobile phones. In such receivers, a radio frequency signal, or a down-converted version thereof, is typically amplified and/or filtered in the analog signal processing before it is converted to the digital domain for further processing. Thus, the relevant building blocks related to this are amplifiers, mixers, filters, and analog-to-digital converters.
The performance of such receivers heavily depends on the analog signal processing, and thus the relevant building blocks need to meet some strict requirements. Among other things, they must show a sufficiently high degree of linearity, so that distortion can be minimized. Even order nonlinearity is non-desired in most electronic systems, and it is especially harmful to the performance of direct conversion receivers. There are thus very stringent requirements throughout the analog signal path on even order linearity in such receivers. It is noted that the most relevant type of even order nonlinearity is second order nonlinearity, and therefore this term is often used instead.
Even order non-linearity is inherent in transistors. The most popular is to use MOS transistors, which are ideally square law devices with a pure second order non-linearity. Bipolar transistors have an exponential characteristic with strong non-linearity.
The even order non-linearity may be reduced or canceled by using differential circuits. The extent of cancelation depends on the matching between the halves of the circuit. However, in the design of cellular receivers, to save cost there is a trend to use single-ended inputs to mobile phone transceivers instead of differential inputs, which reduces the possibilities of using this type of cancellation.
Another recent trend is to reduce the amount of filtering between antenna and receiver, calling for increased linearity. Normally, an antenna filter, e.g. a SAW (surface acoustic wave) filter, has been used between the antenna and the analog signal processing. Removing the antenna filter means that very strong interference will be present at the input of the transceiver chip, which is single-ended. The even order non-linearity will then produce strong low frequency signals in the single-ended part of the receiver, i.e. the low noise amplifier, and if not blocked also strong low frequency common-mode signals in the differential part, i.e. the baseband. These signals will reduce the headroom for the desired signals and should thus be minimized. Even more important, with the antenna filter removed strong out-of-band signals can inter-modulate in the low noise amplifier, and if the intermodulation product is at the same frequency as the signal to receive, reception can be blocked. The intermodulation can occur due to both even and odd order non-linearities.
An amplifier type that can be used in these situations is the push-pull amplifier. A push-pull amplifier is usually implemented with a complementary pair of transistor devices of opposite conductivity type arranged in series between two supply voltages, typically a positive supply voltage and either a negative supply voltage or ground. One of the transistor devices is arranged to supply current to a load from the positive supply voltage, and the other one is arranged to sink current from the load to ground or the negative supply voltage. This amplifier type is interesting, especially because its symmetrical construction with the two transistor devices means that even order harmonics are in principle cancelled, so that even order non-linearity can be avoided or at least reduced. Further, the push-pull amplifier is simple in structure and has low power consumption and a relatively high gain.
If the two complementary transistor devices are designed to have the same analog characteristics, except for their opposite conductivity type, the even order non-linearity of the amplifier stage should be very low or zero. However, during production of the amplifier on an integrated circuit the two transistor devices are typically formed in different process steps, which means that in practice, due to production tolerances, it is difficult to achieve identical analog characteristics for the two complementary transistor devices, and therefore a certain amount of even order non-linearity will still be present in the amplifier stage. Thus, in practice, the push-pull amplifier seems not to be as beneficial to this application as it ideally should be, unless the problem with the production tolerances can be solved.
US 2011/0133839 describes an arrangement for calibrating the quiescent operating point of a push-pull amplifier. In a calibration mode, a control arrangement applies a test signal to the amplifier and measures an even order distortion of the amplifier. Based on the measured even order distortion the control arrangement adjusts a control signal, and a controllable biasing circuit is provided for changing the quiescent operating point of the amplifier as a function of the control signal, so that the even order distortion can be kept below a critical level. This is a complex and expensive solution because the measurement of the distortion directly at the amplifier output involves analog-to-digital conversion and performing a Fast Fourier Transform or similar function to determine the distortion components. Further, dedicated test tones have to be generated, and the normal operation of the amplifier is disturbed by the distortion measurement.